Structural composites are materials that have generally been developed in attempts to combine desirable properties of two or more different types of materials. For example, reinforced plastic materials have been developed where the plastic material is a matrix material and the reinforcement is in the form of fibers or particles dispersed within the matrix material. In such reinforced plastic composite materials, the low density, toughness, and processability of the plastic matrix material is combined with the stiffness, strength, heat resistance, and relatively low cost of such reinforcements as glass fibers or mineral-based fillers. Reinforced concrete is another simple example of a structural composite material, where steel or other metal reinforcement bars (rebar) are aligned within a concrete matrix to impart some of the tensile strength of the rebar to the resulting material, where one of the most undesirable properties of unreinforced concrete is its exceptionally low tensile strength. Typically, such composite materials have overall mechanical properties that are a compromise between the combined materials governed in part by the so-called “rule of mixtures” that takes the contributions of the individual constituents into account. For example, a glass fiber-reinforced plastic is stronger that the plastic matrix, but weaker than the individual glass fibers. It is also tougher than the glass fibers, but not as tough as the plastic matrix. With these types of composites, efforts have been made to increase the matrix-to-fiber adhesion or “wetting” of the fibers with the matrix material so that stresses applied to the matrix material are shared with the stronger fibers.
Ceramic matrix composite (CMC) materials have been developed with similar objectives, but have evolved in different ways to address certain material properties. CMC materials include a ceramic matrix material and a ceramic reinforcement material, where the reinforcement material is typically in the form of long fibers. While the reinforcements are meant to impart some of their strength or other properties to the overall composite material, they are also provided to interrupt crack propagation through the matrix material, where cracking due to fatigue, impact, or thermal shock is a primary weakness of ceramic materials. Such controlled crack propagation is said to effectively “toughen” the matrix material, though the toughening mechanism does not involve the reinforcements themselves absorbing mechanical energy. Rather, mechanical energy is dissipated through relative movement at the matrix-fiber interface. In order for this type of toughening mechanism to work, the adhesion between the matrix material and the reinforcing fibers must be very low. Thus, with CMC materials, efforts are typically made to reduce the bonding between the matrix material and the reinforcing material in order to increase the effective fracture toughness. This can result in significantly lower mechanical properties than would normally be expected by the rule of mixtures and severe degradation of inter-laminar properties.